Higher CO2 Levels in Atmosphere May Speed Soil Emissions

Nathanael Massey

Forests may not help mitigate carbon dioxide pollution thanks to an uptick in CO2 emissions from decomposition

As higher levels of carbon dioxide permeate the Earth's atmosphere, scientists have long counted on forests -- which, as individual trees, grow larger in carbon-rich environments -- to soak up some of the excess.

But after nearly a decade and a half of observing forest ecosystems in controlled settings, scientists now see evidence that elevating carbon levels may cause forests to release as much extra carbon as they absorb.

That's because trees, as they grow larger, need to absorb higher doses of nitrogen and other minerals, as well as the extra carbon dioxide. By feeding some of their excess carbon to microbes in the soil, they can accelerate the decomposition process, gaining access to those important minerals.

As a byproduct of that process, however, more carbon is released into the atmosphere.

"Under elevated CO2, everything is cycling much faster," said Richard Phillips, an assistant professor of biology at the Indiana University College of Arts and Sciences. "The net result is that plants get more nitrogen and store more carbon. But as those microbes cause matter in the soil to break down, carbon is released as a gas into the air."

In the end, more carbon is stored in the wood of the trees, which are growing larger -- but less is stored in the soil, where the decomposition process is taking place.

That finding could have significant implications for climate models, which attempt to factor in long-term carbon storage in forests as part of their overall picture of the global carbon balance.

"In soil, carbon gets worked over by microbes, back and forth, and ends up in these complex compound forms that are very stable," Phillips said. Carbon stored in soil can persist there for hundreds and even thousands of years, he said.

"When carbon is stored in a tree, it's only there for the tree's lifetime -- maybe 60 to 100 years," he added.

Hunting for missing carbon
Scientists had known for some time that elevating carbon dioxide levels in the air didn't necessarily result in more carbon stored in the soil. Free-Air Carbon Dioxide Enrichment (FACE) sites, such as the Duke Forest site where Phillips conducted his research, had for many years documented increased growth in trees without a corresponding rise in soil carbon. As Phillips explained, that indicated to researchers that another process was at work.

"The trees were growing larger, and ordinarily you'd think that bigger plants would mean more detritus, branches and leaves would be falling and adding their carbon to the soil," he said. "But we weren't seeing those levels rise as much as we thought. So somewhere, there was a loss."

It also appeared that the trees were continuing to grow larger, even after more than a decade of high-carbon exposure. Early models had predicted that growth due to higher carbon levels would peak and then plateau again, due to shortages of nitrogen and other minerals.

The fact that trees in the FACE studies have continued to grow at accelerated rates indicated that they were securing increased supplies of nitrogen in tandem with their higher doses of carbon dioxide.

Phillips was the first to propose that the trees, via their roots, might be dumping more sugars and other compounds at the soil, in order to fuel microbial activity and speed up the decomposition process. He termed this hypothesis Rhizo-Accelerated Mineralization and Priming (RAMP).

His findings were published this week in the early online edition of the journalEcology Letters.

The net effects of this process are still unknown, Phillips said, adding that he hopes research in this vein will continue and factor into climate projections.

"Most of [the existing] models have limited representation of roots, and none of them include processes such as priming," he said. "Our results demonstrate that interactions between roots and soil microbes play an underappreciated role in determining how much carbon is stored and how fast nitrogen is cycled."